Barium-neodymium-titanate has been widely used a basis for multilayer capacitor (MLC) dielectrics with COG temperature dependence since 1968, when Bolton and Muhlstadt at the University of Illinois (Ph.D and M.S. Thesis, respectively) discovered that this material combined a relatively high dielectric constant with good temperature stability. Since then, various workers have attempted to identify the composition of the primary phase in this material, with little agreement. Suggested compositions, and the date of publication, are summarized below:
______________________________________ Mol % BaO Nd.sub.2 O.sub.3 TiO.sub.2 ______________________________________ BaO.Nd.sub.2 O.sub.3.3TiO.sub.2 (1981) 20.0 20.0 60.0 BaO.Nd.sub.2 O.sub.3.5TiO.sub.2 (1981) 14.3 14.3 71.4 BaO.Nd.sub.2 O.sub.3.4TiO.sub.2 (1984) 16.7 16.7 66.7 15BaO.19Nd.sub.2 O.sub.3.72TiO.sub.2 (1984) 14.2 17.9 67.9 4BaO.5Nd.sub.2 O.sub.3.18TiO.sub.2 (1986) 14.8 18.5 66.7 ______________________________________
Part of the difficulty in obtaining agreement is that the barium-neodymium-titanate was made by calcining a mechanical mixture of powdered ingredients. Generally, the neodymium oxide that is available commercially has a particle size greater than 10 microns and is difficult to mill to a fine powder, so the degree of reaction with the other ingredients can be variable. One would expect that this problem could be minimized if the compound were made by chemical synthesis, for example by the method described by Colombet and Magnier in U.S. Pat. No. 4,757,037 which issued in 1988. In the work of Colombet and Magnier, a barium-neodymium-titanate of nominal composition BaO. Nd.sub.2 O.sub.3.3TiO.sub.2 was reported to have been made by co-precipitation, after mixing a solution of barium and neodymium nitrates with a titania sol. However, this composition lacked the required temperature stability.
It is usually not possible to achieve the requisite temperature stability with the barium-neodymium-titanate system unless the composition is modified with additives. For example, Kashima and Tomuro, in U.S. Pat. No. 4,522,927 which issued in 1985, describe changes in the Temperature Coefficient of Capacitance (TCC) produced by replacing titanium oxide with zirconium oxide according to the formula: EQU xBaO--yNd.sub.2 O.sub.3 --z(Ti.sub.1-m Zr.sub.m)O.sub.2
where x+y+z=1.00, and 0.05&lt;m&lt;0.25. Small additions of MnO.sub.2, Cr.sub.2 O.sub.3, FeO, NiO or CoO are also suggested. Despite the beneficial effect on TCC of replacing some titanium oxide with zirconium oxide, the compositions disclosed by Kashima and Tomuro was found to yield dielectrics with low insulation resistance at 125.degree. C.
Numerous other modifications to the barium-neodymium-titanate system are described in the prior art but they typically involve the use of bismuth oxide and/or lead oxide. (See, for example, U.S. Pat. No. 4,866,017). Bismuth oxide can react adversely with Pd electrodes in MLC's, and the processing of powders containing lead oxide can introduce health and environmental concerns. Partial substitution of neodymium oxide with samarium oxide (Sm.sub.2 O.sub.3) or praseodymium oxide (Pr.sub.6 O.sub.11) has also been suggested as a means of adjusting the TCC (e.g., U.S. Pat. No. 4,500,942).